Use of Mineral Oils to Reduce Fluid Loss for Viscoelastic Surfactant Gelled Fluids

ABSTRACT

Fluids viscosified with viscoelastic surfactants (VESS) may have their fluid loss properties improved with at least one mineral oil which has a viscosity greater than 20 cps at ambient temperature. The mineral oil may initially be dispersed oil droplets in an internal, discontinuous phase of the fluid. In one non-limiting embodiment, the mineral oil is added to the fluid after it has been substantially gelled in an amount between about 0.2 to about 10% by volume.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application from U.S. Ser. No.11/970,389 filed Jan. 7, 2008, issued Nov. 10, 2009 as U.S. Pat. No.7,615,517, which is a continuation-in-part application of U.S. patentapplication Ser. No. 11/517,688 filed Sep. 8, 2006, issued Mar. 25, 2008as U.S. Pat. No. 7,347,266, which in turn claims the benefit of U.S.Provisional Application No. 60/717,307 filed Sep. 15, 2005.

TECHNICAL FIELD

The present invention relates to gelled treatment fluids used duringhydrocarbon recovery operations, and more particularly relates, in oneembodiment, to methods of improving the fluid loss properties of aqueoustreatment fluids containing viscoelastic surfactant gelling agents usedduring hydrocarbon recovery operations.

TECHNICAL BACKGROUND

One of the primary applications for viscosified fluids is hydraulicfracturing. Hydraulic fracturing is a method of using pump rate andhydraulic pressure to fracture or crack a subterranean formation. Oncethe crack or cracks are made, high permeability proppant, relative tothe formation permeability, is pumped into the fracture to prop open thecrack. When the applied pump rates and pressures are reduced or removedfrom the formation, the crack or fracture cannot close or healcompletely because the high permeability proppant keeps the crack open.The propped crack or fracture provides a high permeability pathconnecting the producing wellbore to a larger formation area to enhancethe production of hydrocarbons.

The development of suitable fracturing fluids is a complex art becausethe fluids must simultaneously meet a number of conditions. For example,they must be stable at high temperatures and/or high pump rates andshear rates that can cause the fluids to degrade and prematurely settleout the proppant before the fracturing operation is complete. Variousfluids have been developed, but most commercially used fracturing fluidsare aqueous-based liquids that have either been gelled or foamed. Whenthe fluids are gelled, typically a polymeric gelling agent, such as asolvatable polysaccharide, for example guar and derivatized guarpolysaccharides, is used. The thickened or gelled fluid helps keep theproppants within the fluid. Gelling can be accomplished or improved bythe use of crosslinking agents or crosslinkers that promote crosslinkingof the polymers together, thereby increasing the viscosity of the fluid.One of the more common crosslinked polymeric fluids is boratecrosslinked guar.

While polymers have been used in the past as gelling agents infracturing fluids to carry or suspend solid particles as noted, suchpolymers require separate breaker compositions to be injected to reducethe viscosity. Further, such polymers tend to leave a coating on theproppant and a filter cake of dehydrated polymer on the fracture faceeven after the gelled fluid is broken. The coating and/or the filtercake may interfere with the functioning of the proppant. Studies havealso shown that “fish-eyes” and/or “microgels” present in some polymergelled carrier fluids will plug pore throats, leading to impairedleakoff and causing formation damage. Conventional polymers are alsoeither cationic or anionic which present the disadvantage of likelydamage to the producing formations.

Aqueous fluids gelled with viscoelastic surfactants (VESs) are alsoknown in the art. VES-gelled fluids have been widely used asgravel-packing, frac-packing and fracturing fluids because they exhibitexcellent Theological properties and are less damaging to producingformations than crosslinked polymer fluids. VES fluids arenon-cake-building fluids, and thus leave no potentially damaging polymercake residue. However, the same property that makes VES fluids lessdamaging tends to result in significantly higher fluid leakage into thereservoir matrix, which reduces the efficiency of the fluid especiallyduring VES fracturing treatments. It would thus be very desirable andimportant to discover and use fluid loss agents for VES fracturingtreatments in high permeability formations.

SUMMARY

There is provided, in one form, a method for reducing the fluid loss ofaqueous fluids gelled with a viscoelastic surfactant (VES) that involvesadding to an aqueous fluid substantially gelled with at least one VES atleast one mineral oil fluid loss control agent in an amount that iseffective to reduce the fluid loss of the gelled aqueous fluid. Themineral oil has a viscosity greater than 20 cps at ambient temperature.

In another embodiment, there is provided an aqueous fluid that includeswater; at least one VES in an amount effective to increase the viscosityof the aqueous fluid; and at least one mineral oil in an amounteffective to reduce the fluid loss of the gelled aqueous fluid. Themineral oil is added to the fluid after the fluid is substantiallygelled. Again, the mineral oil has a viscosity greater than 20 cps atambient temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of leakoff tests using 400 millidarcy (mD) ceramicdiscs at 150° F. (66° C.) and 100 psi (0.7 MPa) plotting leakoff volumeas a function of time, where the base fluid is 3% KCl with 6% VES,without and with 2% mineral oil within the definitions herein; and

FIG. 2 is a graph of viscosities of aqueous fluids (3% KCl) gelled with6% VES at 150° F. (66° C.) and 100 1/s without and with 2% mineral oilwithin the definitions herein.

DETAILED DESCRIPTION

It has been discovered that the addition of certain mineral oils inrelatively small quantities to an aqueous fluid gelled with a VESimproved the fluid loss of these brines. The fluid loss control agentsherein are believed to be particularly useful in VES-gelled fluids usedfor well completion and/or stimulation. The VES-gelled fluids mayfurther comprise proppants or gravel, if they are intended for use asfracturing fluids or gravel packing fluids, although such uses do notrequire that the fluids include proppants or gravel. It is especiallyuseful that the removal of these fluid loss control agents may be easyand complete maintaining little or no damage to the formation. Inparticular, the VES-gelled aqueous fluids with these mineral oils areexpected to have improved (reduced, diminished or prevented) fluid lossover a broad range of temperatures, such as from about 70 (about 21° C.)to about 400° F. (about 204° C.); alternatively up to about 350° F.(about 177° C.), and in another non-limiting embodiment up to about 300°F. (about 149° C.). In some cases suitable reservoir temperatures may bebetween about 100° to about 270° F. (about 37° to about 132° C.). Thesemineral oils may be used alone or together with other fluid loss controlagents such as alkaline earth metal oxides, alkaline earth metalhydroxides, transition metal oxides, transition hydroxides, and amixture thereof. These latter fluid loss control agents are furtherdescribed in U.S. Pat. No. 7,550,143 issued Jun. 23, 2009, incorporatedby reference herein in its entirety. Further, the mineral oils describedherein do not noticeably change the initial viscosity of VES-gelledfluids for at least 90 minutes, which is surprising given that reservoirhydrocarbons are known to break VES-gelled fluids.

This discovery allows the VES system to have improved fluid loss to helpminimize formation damage during well completion or stimulationoperations. That is, the introduction of these additives to theVES-gelled aqueous system will limit and reduce the amount of VES fluidwhich leaks-off into the pores of a reservoir during a fracturing orfrac-packing treatment, thus minimizing the formation damage that mayoccur by the VES fluid within the reservoir pores. Also, limiting theamount of VES fluid that leaks-off into the reservoir during a treatmentwill allow more fluid to remain within the fracture and thus less totalfluid volume will be required for the treatment. Having less fluidleaking off and more fluid remaining within the fracture will enablegreater fracture size and geometry to be generated. Thus the use ofthese additives in a VES-gelled aqueous system will improve theperformance of the VES fluid while lowering fracturing treatment cost.

Prior art VES-gelled aqueous fluids, being non-wall-building fluids(i.e. there is no polymer or similar material build-up on the formationface to form a filter cake) that do not build a filter cake on theformation face, have viscositycontrolled fluid leakoff into thereservoir. By contrast, the methods and compositions herein use a fluidloss agent that forms small oil drops to hinder the waterbased VES fluidflow through the porous medium to reduce fluid leakoff. Surprisingly thesmall oil drops are very compatible with the VES micelle structures inthe fluid and do not significantly reduce or impair the viscosity of VESfluid during the pumping of a treatment.

A new method has been discovered to reduce the leakoff of aqueous fluidsgelled with viscoelastic surfactants (i.e. surfactants that developviscosity in aqueous brines, including chloride brines, by formation ofrod- or worm-shaped micelle structures). The improvement will permitless VES to be used since less of it will leak off into the formation.The fluid loss control agent herein may be added to the gel after batchmixing of a VES-gel treatment, or added onthe-fly after continuousmixing of a VES-gel treatment using a liquid additive metering system inone non-limiting embodiment. The high viscosity mineral oils are notsolubilized in the brine, since they are inherently highly hydrophobic,but initially they are dispersed as microscopic oil droplets. The oildroplets may be understood as dispersed in the “internal phase” as a“discontinuous phase” of the brine medium/VES fluid which is the “outerphase” or “continuous phase”.

Surprisingly and unexpectedly the method employs mineral oils as a fluidloss control component. This is surprising because the literatureteaches that contact of a VES-gelled fluid with hydrocarbons, such asthose of the subterranean formation in a non-limiting example,essentially instantaneously reduces the viscosity of the gel or “breaks”the fluid. By “essentially instantaneously” is meant less than one-halfhour. In general mineral oils are highly saturated hydrocarbons and fromthe literature one would expect VES-micelles to break upon contactingsaturated hydrocarbons.

In one non-limiting embodiment the mineral oil is added before the VESgelling agent. In another non-limiting embodiment herein the mineral oilis added after the aqueous fluid is substantially gelled. By“substantially gelled” is meant that at least 90% of the viscosityincrease has been achieved before the mineral oil is added. Of course,it is acceptable to add the mineral oil after the gel has completelyformed.

Mineral oil (also known as liquid petrolatum) is a by-product in thedistillation of petroleum to produce gasoline. It is a chemically inerttransparent colorless oil composed mainly of linear, branched, andcyclic alkanes (paraffins) of various molecular weights, related towhite petrolatum. Mineral oil is produced in very large quantities, andis thus relatively inexpensive. Mineral oil products are typicallyhighly refined, through distillation, hydrogenation, hydrotreating, andother refining processes, to have improved properties, and the type andamount of refining varies from product to product. Highly refinedmineral oil is commonly used as a lubricant and a laxative, and withadded fragrance is marketed as “baby oil” in the U.S. Most mineral oilproducts are very inert and non-toxic, and are commonly used as babyoils and within face, body and hand lotions in the cosmetics industry.Other names for mineral oil include, but are not necessarily limited to,paraffin oil, paraffinic oil, lubricating oil, white mineral oil, andwhite oil.

In one non-limiting embodiment the mineral oil has a high content ofisoparaffins, and is at least 99 wt % paraffinic. Because of therelatively low content of aromatic compounds, mineral oil has a betterenvironmental profile than other oils. In general, the more refined andless aromatic the mineral oil, the better. In another non-restrictiveversion, the mineral oil may have a distillation temperature above about300° C. In another non-restrictive version, the mineral oil has adynamic viscosity of greater than about 20 cps at ambient temperature.Ambient temperature is defined herein as about 20° C. (68° F.). In analternate, non-limiting embodiment, the kinematic viscosity of themineral oil at 40° C. should be at least about 40 cSt. Specific examplesof suitable mineral oils include, but are not necessarily limited to,PURE PERFORMANCE® 225N and 600N Base Oils available from ConocoPhillips,high viscosity ULTRA-S mineral oils from SOil Corporation, such asULTRA-S 8, and high viscosity mineral oils from Sonneborn RefinedProducts, such as GLORIA®, KAYDOL®, BRITOL® 35 USP, HYDROBRITE® 200,380, 550, 1000, and the like. The dynamic viscosity of PURE PERFORMANCE®225N oil at 40° C. is typically 42.7 cps, and dynamic the viscosity of600N oil is typically 114.5 cps. The use of mineral oils herein is safe,simple and economical.

In one non-limiting embodiment, other refinery distillates maypotentially be used in addition to or alternatively to the mineral oilsdescribed herein, as may be hydrocarbon condensation products.Additionally, synthetic oils, such as hydrogenated polyalphaolefins,saturated fatty acids, and other synthetically derived hydrocarbons maybe of utility to practice this invention.

The amount of mineral oil needed to improve the leakoff properties of aparticular VES-gelled aqueous fluid is dependent upon a number ofinterrelated factors and is difficult to predict in advance. Typically,empirical laboratory work is helpful to determine a suitable proportion.The dynamic viscosity and/or kinematic viscosity, molecular weightdistribution, and amount of impurities (such as aromatics, olefins, andthe like) appear to influence the effect a particular mineral oil willhave on a VES-gelled fluid at a given temperature. The effective amountof mineral oil ranges from about 0.2 to about 10% by (by volume) basedon the total fluid, in another non-limiting embodiment from a lowerlimit of about 0.5% by. Independently the upper limit of the range maybe about 3% by of the total fluid.

The use of the disclosed fluid loss control system is ideal for fluidloss reduction of VES based fracturing fluids. The fluid loss system mayalso be used for improving fluid loss in gravel pack fluids, acidizingor near-wellbore clean-up diverter fluids, and loss circulation pillfluids composed of VES. The fluid loss system may additionally work forfoamed fluid applications (hydraulic fracturing, acidizing, and thelike), where N₂ or CO₂ gas is used for the gas phase. This fluid lossimprovement methods and compositions herein will help conserve thefluids used, and the additives therein, for these various applications.

Any suitable mixing apparatus may be used for incorporating the mineraloil fluid loss additive. In the case of batch mixing, the VES and theaqueous fluid are blended for a period of time sufficient to form agelled or viscosified solution. The mineral oil can be added before orafter the fluid is substantially gelled. The VES that is useful in thepresent invention can be any of the VES systems that are familiar tothose in the well service industry, and may include, but are not limitedto, amines, amine salts, quaternary ammonium salts, amidoamine oxides,amine oxides, mixtures thereof and the like. Suitable amines, aminesalts, quaternary ammonium salts, amidoamine oxides, and othersurfactants are described in U.S. Pat. Nos. 5,964,295; 5,979,555; and6,239,183, incorporated herein by reference in their entirety.

Viscoelastic surfactants improve the fracturing (frac) fluid performancethrough the use of a polymer-free system. These systems, compared topolymeric based fluids, can offer improved viscosity breaking, highersand trans-port capability, are in many cases more easily recoveredafter treatment than polymers, and are relatively non-damaging to thereservoir with appropriate contact with sufficient quantity of reservoirhydrocarbons, such as crude oil and condensate, or with use of internalbreaking agents. The systems are also more easily mixed “on the fly” infield operations and do not require numerous coadditives in the fluidsystem, as do some prior systems.

The viscoelastic surfactants suitable for use in this invention include,but are not necessarily limited to, non-ionic, cationic, amphoteric, andzwitterionic surfactants. Specific examples of zwitterionic/amphotericsurfactants include, but are not necessarily limited to, dihydroxylalkyl glycinate, alkyl ampho acetate or propionate, alkyl betaine, alkylamidopropyl betaine and alkylimino mono- or di-propionates derived fromcertain waxes, fats and oils. Quaternary amine surfactants are typicallycationic, and the betaines are typically zwitterionic. The thickeningagent may be used in conjunction with an inorganic watersoluble salt ororganic additive such as phthalic acid, salicylic acid or their salts.

Some non-ionic fluids are inherently less damaging to the producingformations than cationic fluid types, and are more efficacious per poundthan anionic gelling agents. Amine oxide viscoelastic surfactants havethe potential to offer more gelling power per pound, making it lessexpensive than other fluids of this type.

The amine oxide gelling agents RN⁺(R′)₂O⁻ may have the followingstructure (I):

where R is an alkyl or alkylamido group averaging from about 8 to 24carbon atoms and R′ are independently alkyl groups averaging from about1 to 6 carbon atoms. In one non-limiting embodiment, R is an alkyl oralkylamido group averaging from about 8 to 16 carbon atoms and R′ areindependently alkyl groups averaging from about 2 to 3 carbon atoms. Inan alternate, non-restrictive embodiment, the amidoamine oxide gellingagent is Akzo Nobel's Aromox, APA-T formulation, which should beunderstood as a dipropylamine oxide since both R′ groups are propyl.

Materials sold under U.S. Pat. No. 5,964,295 include CLEARFRAC™, whichmay also comprise greater than 10% of a glycol. One preferred VES is anamine oxide. As noted, a particularly preferred amine oxide is APA-T,sold by Baker Oil Tools as SURFRAQ™ VES. SURFRAQ is a VES liquid productthat is 50% APA-T and greater than 40% propylene glycol. Theseviscoelastic surfactants are capable of gelling aqueous solutions toform a gelled base fluid. One excellent VES system is sold by Baker OilTools as DIAMONDFRAQ™. DIAMOND FRAQ™, which has assured breakingtechnology that overcomes reliance on external reservoir conditions inorder to break, as compared with products such as CLEARFRAC™.

The methods and compositions herein also cover commonly known materialsas AROMOX® APA-T manufactured by Akzo Nobel and other known viscoelasticsurfactant gelling agents common to stimulation treatment ofsubterranean formations.

The amount of VES included in the fracturing fluid depends on at leasttwo factors. One involves generating enough viscosity to control therate of fluid leakoff into the pores of the fracture, and the secondinvolves creating a viscosity high enough to keep the proppant particlessuspended therein during the fluid injecting step, in the non-limitingcase of a fracturing fluid. The additives herein help improve the firstfactor. Thus, depending on the application, the VES is added to theaqueous fluid in concentrations ranging from about 0.5 to 25% by volume,alternatively up to about 12 vol % of the total aqueous fluid (fromabout 5 to 120 gptg). (It will be appreciated that units of gallon perthousand gallons (gptg) are readily converted to SI units of the samevalue as, e.g. liters per thousand liters.) In another non-limitingembodiment, the range for the present formulations is from about 1.0 toabout 6.0% by volume VES product. In an alternate, non-restrictive form,the amount of VES ranges from a lower limit of about 2 independently toan upper limit of about 10 volume %.

It is expected that the fluid loss additives described herein may beused to improve the fluid loss of a VES-gelled aqueous fluid regardlessof how the VES-gelled fluid is ultimately utilized. For instance, thefluid loss compositions could be used in all VES applications including,but not limited to, VES-gelled friction reducers, VES viscosifiers forloss circulation pills, drilling fluids, fracturing fluids (includingfoamed fracturing fluids), gravel pack fluids, viscosifiers used asdiverters in acidizing (including foam diverters), VES viscosifiers usedto clean up drilling mud filter cake, remedial clean-up of fluids aftera VES treatment (post-VES treatment) in regular or foamed fluid forms(i.e. the fluids may be “energized”) with the gas phase of foam being N₂or CO₂, and the like.

In order to practice the methods described herein, an aqueous fracturingfluid, as a non-limiting example, is first prepared by blending a VESinto an aqueous fluid. The aqueous fluid could be, for example, water,brine, aqueous-based foams or water-alcohol mixtures. Any suitablemixing apparatus may be used for this procedure. In the case of batchmixing, the VES and the aqueous fluid are blended for a period of timesufficient to form a gelled or viscosified solution. As noted, the fluidloss additive described herein is added separately after the fluid issubstantially gelled, in one non-limiting embodiment. In anothernon-limiting embodiment a portion or all of the fluid loss additive maybe added prior to or simultaneously with the VES gelling agent.

Propping agents are typically added to the base fluid after the additionof the VES. Propping agents include, but are not limited to, forinstance, quartz sand grains, glass and ceramic beads, bauxite grains,walnut shell fragments, aluminum pellets, nylon pellets, and the like.The propping agents are normally used in concentrations between about 1to 14 pounds per gallon (120-1700 kg/m³) of fracturing fluidcomposition, but higher or lower concentrations may be used as thefracture design required. The base fluid can also contain otherconventional additives common to the well service industry such as waterwetting surfactants, non-emulsifiers and the like. As noted herein, thebase fluid may also contain other conventional additives which may helpimprove the fluid loss characteristics of the VES fluid, and which areadded for that purpose in one non-restrictive embodiment.

In a typical fracturing operation, the fracturing fluid herein may bepumped at a rate sufficient to initiate and propagate a fracture in theformation and to place propping agents into the fracture. A typicalfracturing treatment would be conducted by mixing a 20.0 to 60.0gallon/1000 gal water (60.0 liters/-1000 liters) amine oxide VES, suchas SURFRAQ, in a 3% (w/v) (249 lb/1000 gal, 29.9 kg/m³) KCl solution ata pH ranging from about 6.0 to about 9.0. The fluid loss component maybe added during the VES addition or more typically after the VESaddition using appropriate mixing and metering equipment.

In one embodiment herein, the method is practiced in the absence ofgel-forming polymers and/or gels or aqueous fluids having theirviscosities enhanced by polymers. However, combination use with polymersand polymer breakers may also be of utility. For instance, polymers mayalso be added to the VES fluids for further fluid loss control purposes.Types of polymers that may serve as fluid loss control agents include,but are not necessarily limited to, various starches, polyvinylacetates, polylactic acid, guar and other polysaccharides, gelatins, andthe like.

The present invention will be explained in further detail in thefollowing non-limiting Examples that are only designed to additionallyillustrate the invention but not narrow the scope thereof.

General Procedure for Examples

To a blender were added tap water, the wt % and type of indicated salt,followed by the indicated vol % of viscoelastic surfactant(WG-3L—Aromox, APA-T available from Akzo Nobel). The blender was used tomix the components on a very slow speed, to prevent foaming, for about30 minutes to viscosity the VES fluid. In the samples where mineral oilwas added, the indicated amounts of ConocoPhillips PURE PERFORMANCES225N Base Oil was used. Leakoff tests were performed using a static testmethod and 400md ceramic disc (0.25 inch thick and 2.5 inches indiameter) representing underground porous medium.

Measurements using a Grace 5500 rheometer at the indicated temperaturesat 100 sec⁻¹ were used to acquire quantitative viscosity of each sample.

Example 1

Shown in FIG. 1 are the graphs of two leakoff tests plotting leakoffvolume as a function of time. The base fluid was 3% KCl with 6% VES at150° F. (66° C.) and 100 psi (0.7 MPa), where the test was conductedwith 400 millidarcy (mD) ceramic discs. It is very apparent that thefluid without any mineral oil additive leaked off rapidly, at about 15minutes, where the fluid with 2% mineral oil within the definitionherein leaked off much more slowly. The mineral oil is Pure Performances225N Base Oils available from ConocoPhillips.

Example 2

FIG. 2 is a graph of the viscosities of aqueous fluids (3% KCl) gelledwith 6% VES at 150° F. (66° C.) and 100 1/s without and with 2% mineraloil within the definition herein. It will be seen that the viscositybehavior of the two fluids over the studied time period is almostidentical, which demonstrates that this small amount of mineral oiladditive does not adversely affect the fluid viscosity, which issurprising since larger amounts of hydrocarbons and mineral oils tend toinhibit or break the gel of VES-gelled fluids.

As can be seen, the method of improving fluid loss characteristicsdescribed herein is simple, effective, and safe.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof, and has been demonstrated aseffective in providing methods and compositions for improving the fluidloss properties of a VES fracturing fluid. However, it will be evidentthat various modifications and changes can be made thereto withoutdeparting from the broader spirit or scope of the invention as set forthin the appended claims. Accordingly, the specification is to be regardedin an illustrative rather than a restrictive sense. For example,specific combinations of viscoelastic surfactants, mineral oils,optional additional fluid loss control agent and other componentsfalling within the claimed parameters, but not specifically identifiedor tried in a particular composition or fluid, are anticipated to bewithin the scope of this invention.

The terms “comprises” and “comprising” in the claims should beinterpreted to mean including, but not limited to, the recited elements.

1. An aqueous fluid comprising: water; at least one viscoelasticsurfactant (VES) in an amount effective to increase the viscosity of theaqueous fluid; and from about 0.2 to about 10% by volume (by) based onthe total fluid of at least one mineral oil fluid loss control agent inan amount effective to reduce the fluid loss in a subterranean reservoirof the gelled aqueous fluid, where the mineral oil fluid loss controlagent is added to the fluid before, during or after the VES, and wherethe mineral oil fluid loss control agent has a viscosity greater than 20cps at ambient temperature, and where the viscosity is not reduced inless than one-half hour.
 2. The aqueous fluid of claim 1 where themineral oil fluid loss control agent is at least about 99 wt % paraffin.3. The aqueous fluid of claim 1 where the mineral oil fluid loss controlagent has a distillation temperature above about 300° C.
 4. An aqueousfluid comprising: water; at least one viscoelastic surfactant (VES) inan amount effective to increase the viscosity of the aqueous fluid; andfrom about 0.2 to about 10% by based on the total fluid of at least onemineral oil fluid loss control agent in an amount effective to reducethe fluid loss in a subterranean reservoir of the gelled aqueous fluid,where the mineral oil fluid loss control agent is added to the fluidbefore, during or after the VES, and where the mineral oil fluid losscontrol agent is at least about 99 wt % paraffin, has a distillationtemperature above about 300° C. and has a viscosity greater than 20 cpsat ambient temperature, and where the viscosity is not reduced in lessthan one-half hour.